1. Field of the Invention
This invention relates to the retorting of subsurface carbonaceous deposits and more particularly to the construction of a rubblized in-situ retort in such deposits.
2. Description of the Prior Art
In-situ combustion processes have been proposed for the recovery of valuable products, particularly fuels, from underground deposits of carbonaceous deposits such as petroleum, coal, and oil shale. It is necessary in in-situ combustion processes to pass combustion air and products of combustion through the deposit if combustion is to be maintained. Many of the carbonaceous deposits, particularly those of oil shale and coal, have a very low permeability that makes treatment of the deposit to increase its permeability necessary before an in-situ combustion recovery process can be used. One method of increasing the permeability is to rubblize the deposit. Rubblization is particularly valuable in increasing the permeability of oil shale deposits and for that reason this invention will be described with specific reference to oil shale.
Shale oil is recovered from oil shale by heating the oil shale to a temperature above about 800.degree. F. at which kerogen, a solid carbonaceous deposit in the oil shale, is decomposed at the high temperature to yield shale oil. A number of retorting processes have been developed in which oil shale is mined and lifted to the surface where it is passed through retorts to heat the oil shale to a temperature at which shale oil is liberated.
To avoid the high cost of mining oil shale and lifting it to the surface and to eliminate some problems such as disposal of the spent shale, it has been proposed to retort oil shale in-situ in the shale deposit. The heat necessary to cause decomposition of the kerogen in the oil shale is provided by burning a portion of the carbonaceous material in the oil shale. Because of the very low permeability of oil shale and because kerogen is a solid material that cannot be made to flow until heated to a high temperature, it is necessary to rubblize the oil shale before combustion air and hot products of combustion can be made to flow through the shale deposit to heat the oil shale to a temperature at which shale oil is produced. Ordinarily, the rubblized oil shale has a void space of 10 to 30 percent of the volume of the in-situ retort. To avoid channeling through the retort of combustion air injected into the retort for the in-situ combustion with consequent bypassing of oil shale in the deposit, it is important that the rubblization be accomplished in a manner to provide substantially uniform permeability to the flow of gas through the retort.
A number of different mining techniques have been suggested for the rubblization of oil shale deposits to form in-situ retorts having a permeability suitable for the in-situ combustion process. In U.S. Pat. No. 3,001,776 of Van Poollen, it is stated that the retorts can be formed by well-known mining processes such as sublevel stoping, shrinkage stoping, sublevel caving or block caving. Sublevel caving has been recognized as a particularly advantageous method for forming in-situ retorts because of the close control of void space and rubblization and, consequently, permeability of the retort that is possible with that mining method. Sublevel caving involves the formation of a vertical starting slot at one end of the retort or area to be mined. The starting slot provides void space and an exposed surface that allows free face blasting. After blasting which rubblizes a portion of the rock, broken rock is withdrawn and the blasting is repeated. The blasting operation retreats from the starting slot across the zone to be mined, towards a shaft in which the mined rock is lifted to the surface.
In the rubblization of rock to form an in-situ retort, only a portion of the rock broken by the blasting is withdrawn for a subsequent blasting step to provide void space that imparts the desired permeability to the rubblized zone. Sublevel caving is described in detail in the SME Mining Engineering Handbook, published by the Society of Mining Engineers of The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. in 1973, beginning at page 12-222 and continuing to page 12-233.
One method of utilizing sublevel caving in the formation of an in-situ retort in oil shale is described in U.S. Pat. No. 4,017,119 of Arthur E. Lewis. In that patent, variations in permeability are produced in the rubblized deposit to prevent plugging of the deposit by softened oil shale of high kerogen content. As described in U.S. Pat. No. 4,017,119, the sublevel caving operation is conducted simultaneously at several sublevels of the deposit with the rubblization being most advanced in the upper layer of rubblized oil shale. The starting slot for the mining and rubblization operations at all sublevels is at the same end of the retort, and the mining operations retreat in the same direction on all sublevels.
The blasting of oil shale into the starting slot results in a void space of the rubblized rock in the zone of the starting slot of approximately 40 percent whereas the desired void space in the remainder of the retort is in the range of 10 to 30 percent. The permeability of the rubblized shale in the zone of starting slots is, therefore, substantially higher than the remainder of the retort. Since all of the starting slots for the different sublevels are located at the boundary of the deposit to be mined or the rubblized retort to be formed remote from the shaft through which the mined rock is lifted to the surface, a zone of high permeability extending from the top of the retort to the bottom is formed along that boundary. During subsequent retorting, combustion air will channel through the highly permeable zone and can result in bypassing of substantial shale in the retort.